\documentclass{standalone}
\usepackage{tcolorbox}
\tcbuselibrary{raster}
\tcbuselibrary{skins}
\begin{document}
\begin{tcbraster}[raster columns=3,colframe=gray,colback=white,
    colbacktitle=gray!50!white,fonttitle=\small\bfseries\ttfamily,
    left=0pt,right=0pt,top=0pt,bottom=0pt,boxsep=0pt,boxrule=0.6pt,
    toptitle=1mm,bottomtitle=1mm,drop lifted shadow,center title]
  \tcbincludepdf{<theme>.pdf}
\end{tcbraster}
\end{document}